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closes bpo-28955: Clarified comparisons between NaN and number in reference documentation (GH-5982) 

Co-authored-by: Benjamin Peterson <benjamin@python.org>
(cherry picked from commit ad8a000420)

Co-authored-by: Tony Flury <anthony.flury@btinternet.com>
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Miss Islington (bot) 2018-09-14 11:05:38 -07:00 committed by GitHub
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Doc/reference/expressions.rst
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@ -1341,12 +1341,11 @@ built-in types.
involved, they compare mathematically (algorithmically) correct without loss
of precision.
The not-a-number values :const:`float('NaN')` and :const:`Decimal('NaN')`
are special. They are identical to themselves (``x is x`` is true) but
are not equal to themselves (``x == x`` is false). Additionally,
comparing any number to a not-a-number value
will return ``False``. For example, both ``3 < float('NaN')`` and
``float('NaN') < 3`` will return ``False``.
The not-a-number values ``float('NaN')`` and ``decimal.Decimal('NaN')`` are
special. Any ordered comparison of a number to a not-a-number value is false.
A counter-intuitive implication is that not-a-number values are not equal to
themselves. For example, if ``x = float('NaN')``, ``3 < x``, ``x < 3``, ``x
== x``, ``x != x`` are all false. This behavior is compliant with IEEE 754.
* Binary sequences (instances of :class:`bytes` or :class:`bytearray`) can be
compared within and across their types. They compare lexicographically using